Methods of seasoning timber

Seasoning is the controlled removal of moisture from timber to bring its moisture content to 10-15%. The methods are classified as natural and artificial.

A. Natural seasoning

1. Air seasoning: logs are cut into planks and stacked in a shed, well ventilated and protected from sun, rain and wind.

     (shade roof)
   ===================
   --- boards ---        <- sticker (spacer)
   --- boards ---        <- sticker
   --- boards ---
   ___ ___ ___ ___ ___   <- raised platform
   |  |  |  |  |  |       (300 mm above ground)

• Planks are piled on raised platforms with spacers (stickers) of 25 mm so air circulates.
• It takes 6 months to 2-3 years. It is slow but cheap and gives a uniform result.

2. Water seasoning: logs are submerged in running water for 2-4 weeks, with the thick end facing upstream. The sap is washed away, then the logs are dried in air. It is quick and reduces shrinkage, but may weaken the timber.

B. Artificial seasoning

1. Kiln seasoning: timber is stacked in a closed chamber (kiln) in which hot air with controlled humidity and temperature circulates. Temperature is raised in stages (about 35 C to 80 C) with humidity reduced slowly. Done in 4-10 days (depends on thickness). Controlled, fast, but costly.

   Hot air in -> [ stacked timber ] -> moist air out
        ^                                  |
        +----- Fan, heater, steam sprays --+

2. Chemical (salt) seasoning: timber is soaked in a solution of urea or common salt, which draws moisture from the inner part and slows down surface drying, which reduces cracking. 3. Electrical seasoning: a high frequency current passes through the timber and heats it; very fast but costly (used in special cases). 4. Boiling: timber is boiled in water, then dried in air. 5. Smoke seasoning for small items is used in rural areas.

Comparison